Copper is being used in place of aluminum as a metallization layer in high performance integrated circuit (IC) devices. It is well-known to use Gas Assisted Systems (GAS) in conjunction with a focused ion beam (FIB) machine for exposing and milling an aluminum metallization layer. GAS is accomplished by the introduction of a reactive gas into the FIB chamber (through a gas injector) to enhance a rate at which the FIB removes a dielectric layer or an aluminum layer and provides higher selectivity (better end-point control).
The reaction products are volatile which limits re-deposition and permits fabrication of high aspect ratio structures. Chemicals used in GAS are well established when exposing an aluminum metallization. These aluminum metallization exposure chemicals include Cl2 or I2 for aluminum (Al) and silicon (Si) removal, XeF2 for titanium (W), Si3N4, SiO2 removal, and H2O (water-based chemical) for polyimide, photoresist and other carbon based materials removal.
However, many chemicals used for exposing and milling the aluminum metallization layer cannot be used for exposing and milling a metallization layer that is copper (Cu). For example, halogen-based chemicals, such as Cl2 and I2, are used to enhance aluminum layer metallization milling by at least a factor of ten (10) (compared with the default FIB etch without any chemicals). Unfortunately, these halogen-based chemicals have a spontaneous reaction with copper and results in a loss of etch anisotropy, creating a corrosive-like by-products on the surface of the exposed copper metallization layer.
Research has been done on trying to find suitable chemicals for copper milling by FIB vendors, universities and semiconductor companies. Many supplemental chemicals have been suggested and examined trying to find a suitable chemical for copper milling. These supplemental chemicals have included a mixture of Cl2 and anhydrous ammonia (NH3) by K. Edinger, Gas Assisted Etching of Copper with Focused Ion Beams, K. Edinger, Institute for Plasma Research, University of Maryland, College Part, Md., 20742, or C2Cl4 by Phillips, Developments for Chemically Enhanced Focused Ion Beam Micromachining of Copper, J. R. Phillips, T. S. Stark, D. P. Griffis, P. E. Russel; Materials Science and Engineering Dept., Analytical Instrumentation Facility, NCSU, Raleigh, N.C., USA. However, no ideal solution was found so far without compromise on the performance and ease of use. Some of the solutions have safety issues for actual use in an industrial environment.
Additionally, IC manufacturing companies have significant investments in existing FIB machine systems designed for exposing and milling aluminum metallization layers and it would be desirable to use these systems for exposing and milling copper metallization layers with minimal changes. There are two major issues that are addressed for copper metallization layers. These include: 1) exposing copper traces uniformly for cutting or deposition, and 2) milling exposed copper traces using chemicals that uniformly and effectively remove top copper traces without degrading other nearby structures.
Conventional FIB machine systems operate simply when exposing/milling aluminum metallization layers. The conventional FIB system uses a dielectric-enhanced milling chemical, such as commercially available XeF2, to remove the dielectric material overlying a portion of the aluminum trace to be exposed/milled (such as, Si3N4 and/or SiO2). Typically, copper metallization layers are used in high performance ICs that often use C4 (controlled collapse chip connection) flip-chip technology. C4 ICs often have an additional polyimide layer on top of the dielectric material for further protection that is not effectively removed with XeF2.
A polyimide layer may be removed with Selective Carbon Mill (SCM) chemical that has an enhanced rate of 20× relative to the removal of the typical underlying dielectric material. SCM is a special water-based chemical developed for removing carbon-based materials effectively and is available commercially from various vendors, particularly FIB Machine vendors. For example, FEI Company, 7451 NW Evergreen Parkway, Hillsboro, Oreg. USA 97124-5830, phone: 503-640-7500 (http://www.feic.com) is one source for SCM. The SCM chemical does not have any appreciable enhancement for removal/milling for the dielectric layers and metallization layers.
Conventionally, FIB machines use XeF2 for removing both polyimide and dielectric layers, with an enhance rate of 4–15× and 5–10× respectively. However, the milling uniformity of XeF2 strongly depends upon a direction of gas flux induced to a surface of the sample.
FIG. 1 is a schematic illustration of a typical consequence of use of XeF2 in a GAS system 100 having an ion beam column 105 directing a focused ion beam (FIB) 110 in the presence of XeF2 delivered by a gas injector 115. FIB 110 removes a polyimide layer 120 and a dielectric layer 125 overlying a desired node 130. Using only XeF2 produces a trench 135 having one side 140 over-exposed while another side 145 is under-exposed. This asymmetric trench 135 causes potential problems for subsequent work in the area on desired node 130.
FIG. 2 is a schematic illustration of a typical consequence of use of SCM in a GAS system 200 having an ion beam column 105 directing a focused ion beam (FIB) 110 in the presence of SCM delivered by a gas injector 115. FIB 110 removes a polyimide layer 120 and a dielectric layer 125 overlying a desired node 130. Using only SCM produces very long etch times due to slow etch rate of SCM chemical on dielectric layer 125 while producing a trench 135. Additionally, the SCM chemical has a grain preferential etch for copper (Cu) (with some grains etching much faster than others), that damages an exposed copper node 205.
FIG. 3 is a schematic illustration of a typical consequence of use of XeF2 in a GAS system 300 having an ion beam column 105 directing a focused ion beam (FIB) 110 in the presence of XeF2 delivered by a gas injector 115 during a copper milling step. After a polyimide layer 120 and a dielectric layer 125 overlying a desired node 130 has been removed, node 130 is milled using XeF2 in GAS 300. Using only XeF2 for milling produces very long etch times due to slow etch rate of XeF2 chemical on the copper of node 130 while producing a trench 205. One node 130 is completely milled (node 130 is shown in phantom as node 310 after milling), trench 305 is over-etched to expose a non-desired node 315.
Accordingly, what is needed is a system and method for exposing and/or milling a copper metallization layer disposed in a dielectric material that may have an overlying polyimide layer. The system and method should be easily implemented, cost effective and compatible with existing FIB machine systems used for exposing/milling aluminum metallization layers. The present invention addresses such a need.